Zend certified PHP/Magento developer

How to solve Magento 2 “Refuse to apply css because of its MIME type” on Nginx?

I have a Magento 2 store and everything was ok with the default theme. Then I installed Porto theme and now I get this errors in the console and the page (especially the top menu) doesn’t load properly:

The Content-Security-Policy directive 'frame-ancestors' does not support the source expression ''unsafe-inline''

Refused to apply style from 'https://example.com/media/porto/configed_css/settings_default.css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.

As a web server I’m using Nginx.

I tried running this commands after theme installation but I’m still getting the same errors:

php bin/magento setup:upgrade
php bin/magento setup:static-content:deploy -f
php bin/magento indexer:reindex 
php bin/magento cache:flush

Any ideas on what I could further do to fix this?

Thank you!